Colorectal cancer Outcomes in people with Severe Mental Illness Cohort (COSMIC): a protocol for an Australian retrospective cohort using linked administrative data

Introduction Colorectal cancer (CRC) mortality is significantly higher in those with severe mental illness (SMI) compared with the general population, despite similar incidence rates, suggesting that barriers to optimal screening and cancer care may contribute to disparities in CRC mortality in those with SMI. This study aims to compare participation in Australia's National Bowel Cancer Screening Programme (NBCSP) in those with SMI and those in the general population. We will also investigate treatment pathways after diagnosis to determine whether treatment variations could explain differences in CRC mortality. Methods and analysis We will undertake a retrospective cohort study of Australians using linked administrative data to assess differences in screening and cancer care between those with and without SMI, aged 50-74 years on or after 1 January 2006. People with SMI will be defined using antipsychotic medication prescription data. The comparison group will be people enrolled in Medicare (Australia's universal healthcare system) who have not been prescribed antipsychotic medication. Data on outcomes (NBCSP participation, follow-up colonoscopy, CRC incidence and CRC-cause and all-cause mortality) and confounders will be obtained from national-based and state-based administrative health datasets. All people in New South Wales, aged 50-74 with a new diagnosis of CRC on or after 1 January 2006, will be ascertained to examine stage at diagnosis and cancer treatment in those with and without SMI. Poisson regression will be used to calculate incidence rates and rate ratios for each outcome. Ethics and dissemination Ethics approval has been obtained from the University of Queensland Human Research Ethics Committee, the Australian Institute of Health and Welfare Ethics Committee and data custodians from every Australian State/Territory. Findings will be disseminated via publications in peer-reviewed journals and presented at appropriate conferences.